In recent years, following an increase in the degree of density and integration of hybrid ICs and printed circuit boards that carry transistors, ICs, memory elements, and other electronic components, various thermally conductive silicone compositions have been used in order to achieve efficient heat release from such devices. Publicly known thermally conductive silicone compositions include thermally conductive silicone greases, thermally conductive silicone gel compositions, and thermally conductive silicone rubber compositions.
Examples of thermally conductive silicone compositions that have been proposed include, for instance, a thermally conductive silicone composition comprising a vinyl-containing organosiloxane, an organohydrogenpolysiloxane, a thermally conductive filler, an adhesion promoter selected from aminosilane, epoxysilane, or alkyl titanate, and a platinum catalyst (see Japanese Unexamined Patent Application Publication No. Sho 61-157569), a thermally conductive silicone composition comprising an organosiloxane containing an average of two alkenyl groups per molecule, an organosiloxane containing an average of three or more silicon-bonded hydrogen atoms per molecule, a thermally conductive filler made up of zinc oxide and magnesium oxide, a filler treating agent, and a platinum catalyst (see Japanese Unexamined Patent Application Publication No. Sho 62-184058), a thermally conductive silicone composition comprising an organosiloxane containing at least 0.1 mol % alkenyl groups per molecule, an organohydrogenpolysiloxane containing at least two silicon-bonded hydrogen atoms per molecule, a spherical alumina powder with an average particle size of 10 to 50 μm, a spherical or non-spherical alumina powder with an average particle size of less than 10 μm, and platinum or a platinum compound (see Japanese Unexamined Patent Application Publication No.63-251466), a thermally conductive silicone composition comprising an alkenyl-containing organosiloxane, an organohydrogenpolysiloxane, an irregular-shaped alumina powder with an average particle size of 0.1 to 5 μm, a spherical alumina powder with an average particle size of 5 to 50 μm, and a platinum catalyst (see Japanese Unexamined Patent Application Publication No. Hei 2-41362), a thermally conductive silicone composition comprising an organosiloxane containing at least two silicon-bonded alkenyl groups per molecule, an organohydrogenpolysiloxane containing at least three silicon-bonded hydrogen atoms per molecule, a thermally conductive filler with an average particle size of 5 to 20 μm, an adhesion promoter, and platinum or a platinum compound (see Japanese Unexamined Patent Application Publication No. Hei 2-97559).
To increase the thermal conductivity of such thermally conductive silicone compositions, the content of the thermally conductive fillers in the compositions has to be increased. This, however, brings about problems in terms of the handleability and moldability of the resultant silicone compositions.
In addition, other examples of thermally conductive silicone compositions that have been proposed include, for instance, a thermally conductive silicone composition comprising an organosiloxane containing at least two silicone-bonded alkenyl groups per molecule, an organohydrogenpolysiloxane containing at least two silicon-bonded hydrogen atoms per molecule, an organosiloxane containing at least one silicon-bonded alkoxy group or silicon-bonded hydroxyl group per molecule, a spherical or non-spherical alumina micropowder with an average particle size of less than 10 μm, a spherical or non-spherical alumina micropowder with an average particle size of 10 to 50 μm, and a hydrosilation reaction catalyst (Japanese Unexamined Patent Application Publication No. Hei 8-325457), a thermally conductive silicone composition comprising liquid silicone, at least one thickener selected from zinc oxide, alumina, aluminum nitride, boron nitride, or silicon carbide, an organosiloxane having at least one hydroxyl group directly bonded to a silicon atom per molecule, and an alkoxysilane (Japanese Unexamined Patent Application Publication No. Hei 11-49958).
However, in such thermally conductive silicone compositions, the organosiloxane containing at least one silicon-bonded hydroxyl group per molecule is substantially a diorganosiloxane having both terminal ends of its molecular chain blocked by silanol groups. When the amount of the thermally conductive fillers in the composition is increased in order to improve the thermal conductivity of the cured silicone product obtained by curing such a diorganosiloxane, the handleability and moldability of the resultant silicone composition deteriorates.
Furthermore, Japanese Unexamined Patent Application Publication No. 2000-256558 and Japanese Unexamined Patent Application Publication No. 2001-139815 have suggested using a dimethylpolysiloxane represented by the formula:
(where x is an integer of 5 to 100) in thermally conductive silicone compositions.
However, when such a thermally conductive silicone composition is loaded with high levels of alumina or a similar thermally conductive fillers in order to improve the thermal conductivity of the cured silicone product obtained by curing such a composition, the viscosity of the resultant composition rapidly increases and its handleability and moldability markedly deteriorate.
As a result of in-depth investigations into the above-described problems, the present inventors arrived at the present invention.
Namely, it is an object of the present invention to provide a thermally conductive silicone composition exhibiting excellent handleability despite containing a large quantity of thermally conductive fillers added to obtain a silicone composition exhibiting high thermal conductivity.